1. Field of the Invention
This invention relates to transistors, and particularly to transistors having a novel layout resulting in improved electrical and heat dissipation characteristics.
2. Related Art
Transistors have become a common component in modern electrical equipment. One application in which transistors play a vital role is in cellular telephone transmitters and other communication devices. Cellular telephone transmitters send signals from the cellular telephone to a base station. The transmitters use amplifiers to send the signals and the amplifiers include transistors.
Cellular phones process signals prior to and during transmission. The signals may be generated by a voice speaking into a microphone. Cellular phones process the signals at very low currents because low current signals are more efficient to process and allow for longer battery life. However, low current signals must be amplified to high current signals prior to being sent from the cellular telephone to the base station. Transistors perform this current amplification function. One type of transistor that is often used in cellular telephones is a heterojunction bipolar transistor (HBT).
There is a constant need in the industry to create smaller and more efficient transistors with better electrical characteristics. The area occupied by transistors in an amplifier is a significant portion of the total die area of the amplifier. Reducing the transistor die area can substantially reduce the total die area occupied by the amplifier. A reduction of the total die area reduces the cost to manufacture the amplifier. Reducing the die area also allows for increased integration (i.e., an increase in the number of circuit elements that can be integrated into a circuit design). Increased integration allows for smaller cellular phones and cellular phones with greater features.
However, the ability to reduce the transistor die area is hindered by heat dissipation concerns. Heat dissipation is related to the power density of the transistor. Transistors include three main components: a base, an emitter, and a collector. Heat tends to build up under the emitter portion of the transistor. Heat build up is increased as cellular telephone transistors migrate to lower voltages. As the voltage of the cellular telephone system decreases, designers are forced to make transistors larger to accommodate a fixed power requirement (i.e., P=VI, where P is the power, V is the voltage, and I is the current). If V decreases, such as in a low operating voltage environment, then the current I must increase to keep P constant. To prevent heat from building up, an increase in I is accomplished through a larger transistor area and, in particular, a larger emitter area. Therefore, conventional power transistors designed for low voltage systems, such as cellular phones, have layouts with large area requirements in order to handle the high current levels.
One of the primary electrical problems associated with transistors is a susceptibility to emitter failure. In conventional transistors the current density in the emitters is uneven. Generally, there is a higher current density near the input of the emitter and a lower current density away from the input of the emitter. Since a high current density exposes the input area of the emitter to high temperatures (i.e. hot spots) the transistor is susceptible to failure due to emitter burnout.
Another electrical problem associated with transistors is impairment due to high base-to-collector capacitance. High base-to-collector capacitance is associated with the layout of the transistor. For example, base-to-collector capacitance is increased if there is close proximity between the collector and the base. In such layouts, the base and collector become capacitively coupled causing feedback between the collector and base. Furthermore, since the base-to-collector capacitance is proportional to the portions of the base and collector that overlap, large base and collector areas of conventional power transistors increase the base-to-collector capacitance, thereby decreasing performance. High base-to-collector capacitance reduces the current gain of the transistor, leads to radio frequency instability, and leads to problems with signal linearity.
A third electrical problem associated with transistors is a high collector-to-emitter resistance. The high collector-to-emitter resistance problem is also associated with the layout of conventional transistors. High collector-to-emitter resistance causes power loss and power dissipation.
Accordingly, there is a need for a transistor that occupies less area, is suitable for low operating voltages, dissipates heat efficiently, and reduces electrical problems, while maintaining performance and reliability of the transistor.
The invention provides a novel layout for a transistor. The novel layout results in better electrical and heat dissipation characteristics.
Briefly described, in architecture, one embodiment of the novel layout for a transistor, among others, includes an open circular emitter, a collector, a collector contact, and a base pedestal. The open circular emitter has an external border. The collector includes a collector layer portion. The collector contact has an interior border contoured to the emitter exterior border. The collector layer portion has an exterior border contoured to the emitter exterior border. Further, the base pedestal has an exterior border contoured to the emitter exterior border. The transistor may further include a base contact disposed in an emitter void. The base contact may have an exterior border contoured to an emitter interior border. Another embodiment includes emitters of various shapes with other transistor components contoured to the external borders of the various shaped emitters.
Another embodiment includes a transistor with an emitter ballast resistor and wider leg portions and a key-shaped fill portion for additional heat shunting capabilities. Another embodiment omits the emitter ballast resistor and includes additional heat shunting capabilities through an emitter terminal portion of a second metal layer. Still further embodiments include additional heat shunting across a plurality of transistors.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.